The introduction of multiple colour-variable and dimmable lighting devices, in professional retail environments as well as the home environment, has opened possibilities of interactive lighting atmosphere creation. Moreover, it has become essential to enable intuitive user interaction with the lighting system since this often comprise a considerable number of light sources on the ceiling or the walls. To this end, one may utilise lighting devices which comprise both a main light source and a modulated light source, the latter of which can embed identification data in its emitted light. Since high-power light sources which allow modulation are becoming increasingly available, embodiments where the main light source embeds the identification data are often preferred to lighting devices with two separate light sources.
An invention which entails embedding of identification data into light for lighting purposes is described in WO 2006/111934. The identification data, embedded in the light, enable both identification of individual light sources and estimation of their corresponding contributions to the actual colour and intensity of the emitted light seen from different locations. Status data can be transmitted in addition to identifiers. According to the disclosed invention, the contributions from the different light sources are measured with a combined sensor and user-input device. The measured value is subsequently input to a master controller, which generates driving signals provided to the light sources.
In many applications, particularly when the lighting device operates in a retail environment, it is problematical to install permanent light sensors. This is why portable user-input devices, which are primarily an interface by which to specify the desired light output, often are given the additional functionality of measuring the actual light output. The user-input device then constitutes the sole source of feedback to the control device, which provides driving signals to the light sources. One realises that under these circumstances, measurements are made at comparatively long time intervals or altogether irregularly, whenever the user notices that the performance of the lighting device has deteriorated, in other words, that the light atmosphere created by the device has altered visibly. One further realises that the quality of the measurements is all the more critical as they are scarce. An important source of error is misalignment of the user device.
Commercially available light sensors comprise filtered or unfiltered photodetectors, and the vast majority of these are planar, that is, their light-sensitive portion is a flat surface. The normal direction of this surface defines a preferred direction for incident light rays, and consequently planar photodetectors are intrinsically directional, in contrast to radio antennas and other receivers outside the visible range. Likewise, most illuminated objects have a preferred viewing direction—vertical for floors, horizontal for walls and normal for general sloping surfaces—which has to be respected when a corresponding lighting device is designed and arranged in its environment. It is equally important during operation of the lighting device that the light sensor providing information for feedback control is correctly aligned. Regrettably, oblique measurements made by untrained users can be very inaccurate, making rational control of the light sources impossible.
In atmosphere creation, a further problem arises in addition to the mentioned defective reliability of absolute measurements on a single light source. In this technology, there is a frequent need for measurements of relative contributions, for instance, the contribution from each light source (which can be individually identified thanks to the modulation) which is visible from a given point on the illuminated surface. Since obliquely impinging light rays contribute to the brightness of a non-glossy surface by a relatively smaller amount than normal rays, the sought measurement is fundamentally directional, and the issue of correctly aligning the light sensor cannot be ignored. In the particular case of light sources arranged on the ceiling, a planar photodetector would be correctly aligned when it is parallel to the ceiling. Replacing a directional light sensor by an isotropic one in this situation is not a way around the step of aligning the sensor, as this would not reproduce the composite light pattern reaching the point on the surface.